Subsolidus behavior of niobian rutile from the Písek region, Czech Republic: a model for exsolution in W- and Fe2+>>Fe3+-rich phases
Niobian rutile with substantial W content is associated with black tourmaline, beryl, zircon, xenotime-(Y), monazite-(Ce), "písekite", and possibly pseudorutile or pseudobrookite in two cogenetic, beryl-columbite-subtype, granitic pegmatites at Písek and Údraž, southern Bohemia, Czech Republic. Crystals, up to 3×1 cm in size, consist of relics of a primary, phase-homogeneous but oscillatorilly zoned niobian rutile phase and of exsolution intergrowths of depleted niobian rutile with granular titanian-tungstenian ixiolite. Primary niobian rutile has the monorutile structure and shows extensive substitution (Fe, Mn, Mg)2++1 (Nb, Ta)5++2 Ti4+-3 (up to 26.34 wt. % Nb2O5 and 8.60 wt. % Ta2O5; 26 to 37 mol. % columbite component), significant (Fe, Mn, Mg)2++1 W6++1 Ti4+-2 (up to 3.12 wt. % WO3) and minor (Fe, Sc)3++1 (Nb, Ta)5++1 Ti4+-2 substitutions. The values of Fe2+/(Fe2++ Fe3+) are high and average at ~ 0.8. Oscillatory zoning is controlled by Nb, Ta and Fe; variations in W are independent and erratic. Depleted niobian rutile still contains 8 to (rarely) 32 mol. % ferrocolumbite component. Individual grains of the exsolved orthorhombic ixiolite are commonly heterogeneous in terms of the W content. They display highly variable degree of the substitution Fe2++1W6++3Nb5+-4 (up to 49.00 wt. % WO3), moderate but widespread (Ti, Zr, Hf)4++3 (Fe, Mn, Mg, Ca)2+-1 (Nb, Ta)5+-2 (up to 9.83 wt. % TiO2, 1.94 wt. % ZrO2), and subordinate (Fe, Sc)3++3 (Fe, Mn, Mg, Ca)2+-2 (Nb, Ta)5+-1 (up to 1.51 wt. % Sc2O3). Exsolution reveals the preference of Sn4+ and Fe3+ for the rutile structure, whereas Fe2+, Mn, Mg, Nb, Ta, Sc, Zr, Hf and W are concentrated in the ixiolite. The results confirm the extent of the W substitution in pegmatitic niobian rutile and its significance for the style of subsolidus breakdown, on par with the magnitude of effects exerted by (Nb > Ta + Fe2+), (Nb > Ta + Sc + Fe3+), (Nb > Ta + Fe2+/Fe3+~1), or (Fe/Nb > 0.5).
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
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